To me...it it seems "obvious" shooter had a grander and yet "uncompleted plan"....
I have read a lot and been keeping track: 1) The security guard went to the 32nd floor because a "stairwell door was left ajar" and triggered an alarm. 2) The door was ajar because SP was holding it open as he drilled holes into which he would insert pieces of metal to stop the doors from opening. (see linked example of what I mean)
https://i.pinimg.com/236x/49/97/c5/...9db4c59beca--kreg-tools-woodworking-tools.jpg
3) Apparently, an alarm goes off if the door stays open for longer than the time it takes for just ordinary egress.
4) Is that event....the security guard checking out a silent alarm, an open door (isn't that how it always happens?) that stopped SP's plan to kill for hours, undetected, by multiple means and in multiple locations?
This is all I can figure, so far from today's new timeline....
subject to change!